Pulse detonation firing detuning and frequency modulated firing

ABSTRACT

An aircraft engine is provided with at least one pulse detonation device, and the operational frequency of the pulse detonation device is varied over an operational range of frequencies around a mean frequency value. The pulse detonation device can be positioned upstream, downstream or adjacent to a turbine section of the engine. An additional embodiment of the present invention is an aircraft engine provided with more than one pulse detonation device, and the operational frequency of one, or more, of the pulse detonation devices is varied over an operational range of frequencies around a mean frequency value.

BACKGROUND OF THE INVENTION

This invention relates to a method and apparatus for actively varying ormodulating the operating frequency of a pulsed detonation combustor inorder to avoid exciting natural resonant frequencies of an enginesystem.

In many applications, and especially in aircraft engines, excessiveforced response in propulsion or power conversion equipment is avoidedby resonant frequency avoidance. Namely, the driving frequency from somedriving force or combustion source is purposely chosen to not coincidewith a structural resonance or acoustic resonance, of the surroundingstructure. In most cases, the final design of structure or hardwarecontains a fixed set of natural resonant frequencies. In the designprocess, one of the objectives is to make sure that these naturalresonant frequencies do not coincide with any driving power sourcefrequencies.

In the case of reciprocating engines, axial or centrifugal flowpropulsion engines and axial or centrifugal flow power conversionengines the forcing frequency is a function of engine speed. Thus,designers typically avoid operating at speeds where the forcing functionand natural frequencies coincide.

With the advent of the use of pulse detonation devices in propulsionapplications, particularly aircraft propulsion, this consideration mustalso be addressed as pulse detonation devices create an impulsive loadto the chamber and adjacent components. This load can excite resonantmodes in structures. Additionally, fixing the frequency of pulsedetonation devices can provide additional coupling to structuralresonant tones such that excitation exceeds component damping leading toincreased or accelerated component fatigue and/or failure of componentsand structure.

Up until the advent of the present invention, this problem has not beenaddressed with pulsed detonation or quasi-detonation combustion sources.For traditional power sources, firing frequencies are fixed to engineoperating speeds and structures are chosen to avoid resonance coupling.

SUMMARY OF THE INVENTION

In an embodiment of the present invention, and aircraft engine isequipped with at least one pulse detonation device, and the operationalfrequency of the pulse detonation device is varied over an operationalrange of frequencies around a mean frequency value. As discussedpreviously, since the detonation process, of pulse detonation devices,creates an impulsive load to the chamber and adjacent components, andthe impulsive load can excite resonant modes in structures, it isdesirable to detune this driving excitation away from acoustic ormechanical resonances. An embodiment of the present invention isoperated by detuning sets of pulse detonation devices and/or usesfrequency modulation of an individual tube(s) to allow for thedecoupling of the pulse detonation device operational frequencies withstructural natural resonant frequencies and acoustic frequencies.

Additionally, the frequency modulation of the present invention may beused to aid in reducing the levels of ambient noise during engineoperation. Specifically, often during engine operations ambient noise iscreated which increases the overall sound level of engine operations. Inmany applications, including aviation, this additional noise isundesirable due to existing noise regulations and the adverse effect ithas in passenger comfort. This is especially the case during takeoff andlanding operations. The present invention provides a method of activenoise cancellation and control.

As used herein, a “pulse detonation device” (“PDD”) is understood tomean any combustion device or system where a series of repeatingdetonations or quasi-detonations within the device cause a pressure riseand subsequent acceleration of the combustion products as compared tothe pre-burned reactants. A “quasi-detonation” is a combustion processthat produces a pressure rise and velocity increase higher than thepressure rise produced by a deflagration wave. Typical embodiments ofPDDs include a means of igniting a fuel/oxidizer mixture, for example afuel/air mixture, and a confining chamber, in which pressure wave frontsinitiated by the ignition process coalesce to produce a detonation wave.Each detonation or quasi-detonation is initiated either by externalignition, such as spark discharge or laser pulse, or by gas dynamicprocesses, such as shock focusing, autoignition or by another detonationvia cross-firing. The geometry of the detonation chamber is such thatthe pressure rise of the detonation wave expels combustion products outthe PDD exhaust to produce a thrust force or produce power by spinning adownstream turbine. As known to those skilled in the art, pulsedetonation may be accomplished in a number of types of detonationchambers, including detonation tubes, shock tubes, resonating detonationcavities and annular detonation chambers.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrative embodimentof the invention which is schematically set forth in the figures, inwhich:

FIG. 1 is a diagrammatical representation of an aircraft enginecontaining a pulse detonation device, according to one embodiment of thepresent invention; and

FIG. 2 is a diagrammatical representation of an aircraft enginecontaining a plurality of pulse detonation devices, according to anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be explained in further detail by makingreference to the accompanying drawings, which do not limit the scope ofthe invention in any way.

FIG. 1 is a simplified side view of an aircraft engine 100 containing apulse detonation device 104, operated according to an embodiment of thepresent invention. FIG. 2 is a simplified side view of an aircraftengine 100, in accordance with another embodiment of the presentinvention, containing a plurality of pulse detonation devices 104,operated according to another aspect of the present invention.

Turning now to FIG. 1, an exemplary embodiment of the present inventionis shown. In this embodiment an aircraft engine 100 contains a turbinesection 102 and a pulse detonation device 104. The pulse detonationdevice is positioned upstream of the turbine section 102, so that thepulse detonation device 104 acts as a pulse detonation combustor. Thisconfiguration allows the pulse detonation device 104 to acts as theengine combustor, where the exhaust F from the pulse detonation deviceis directed to the turbine 102.

This configuration provides advantages over conventional combustorsystem due to the efficiency of the combustion when using pulsedetonation devices. However, the present invention is not limited tothis exemplary embodiment, as in an additional embodiment the pulsedetonation device 104 is positioned downstream or adjacent to theturbine section 102.

In the present invention, the pulse detonation device 104 is configuredand operated as any known or similar type pulse detonation devices.However, the operational frequency or firing frequency of the pulsedetonation device is modulated so as to avoid providing an impulsiveload which excites any resonant modes in adjacent structural components.

In the complex structure of aircraft engines, and other propulsion orpower generation devices, there are many forcing frequencies which areto be avoided. In particular, it is desirable to avoid naturalfrequencies of structures, acoustic resonance, blade and disk modes inaxial or centrifugal turbines or critical rotor dynamic modes. These,and other critical hardware frequencies, are to be avoided for anyextended period of time.

The embodiment of the present invention, shown in FIG. 1, avoids this byoperating at a fixed frequency until a critical hardware frequency isapproached. As the critical hardware frequency is approached, theoperational frequency of the pulse detonation device is changed. Thechange in frequency can be either step changed or continuously changed.

Additionally and alternatively, the present invention may be used as amethod to control ambient noise generated by the engine. Specifically,the frequency modulation of the present application may be used as ameans of active ambient noise cancellation or control. To achieve thisbenefit, the overall operation and construction of the present inventionremains the same. However, the frequencies and modulation rates may bevaried to achieve the desired affects. The specific frequencies andrates of change for the frequency modulation are to be optimized basedon the desired operational characteristics and goals.

For example, in a non-limiting embodiment of the present invention, themean operational frequency for the pulse detonation device 104 is 200Hz, and the operational frequency of the device 104 continuouslychanged, via the device control system (not shown) between the range of195 and 205 Hz, in increments of at least 1 Hz. In another embodiment,the range of change can be larger such that the pulse detonation devicestep-changes between 195, 200 and 205 Hz, at 5 Hz steps. In a furtherembodiment, the operational frequency is changed in increments of atleast 0.1 Hz. In the present invention, the incremental change amount isselected to ensure that there is no excitation of the acoustic ormechanical resonances, and based on the desired operational parametersand specifications.

The specific operational frequencies discussed above are exemplary andthe present invention is not limited by these frequencies in any way.

In an alternative embodiment, the operational frequency change of thedevice 104 can be controlled such that the change is random. In thisembodiment, a frequency range is determined (for example between 195 and200 Hz) and the operational frequency of the device is randomly changedwithin this range.

In any of the above embodiments, the frequency can be changed afterevery detonation of the device 104, such that any two consecutive cycleswill have a different frequency. In an alternative embodiment, the pulsedetonation device 104 can be operated at a frequency for a predeterminedperiod of time or cycles, and then have the operating frequency changedto another frequency for the same period of time or cycles. In thisembodiment, the number of cycles or amount of time between cycle changesis to be chosen so as to avoid the creation of any excessive impulsiveloads on the structure.

In a further embodiment, the period of time or cycles set at a frequencycan be changed randomly between frequency changes. Stated differently,instead of having the same number of cycles (or time) between frequencychanges, the amount of time or number of cycles between changes can bechanged randomly. Similar to the above embodiment, in this embodimentthe frequency changes are controlled such that even though the durationbetween changes is random, it is controlled so that the longest durationbetween a change is not to exceed a set period or number of cycles,again to avoid creating any undesirable impulsive loads on thestructure.

In the present invention, the control of the firing of the pulsedetonation device, and its firing frequency, can be controlled be anycommonly known control system.

In FIG. 2, a further embodiment of the present invention is shown. Thisembodiment is similar to the embodiment shown in FIG. 1 except that theengine 100 contains more than one pulse detonation device 104. Again,the pulse detonation devices are shown upstream of the turbine section102, to acts as the engine 100 combustor. However, the present inventionis not limited to this configuration, and the pulse detonation devices104 can be located downstream or adjacent to the turbine section.

As with the embodiment shown in FIG. 1, the operating frequencies of thepulse detonation devices 104 are controlled such that their frequenciesare changed through an operating range.

In a further embodiment, each of the pulse detonation devices 104 arefired at a fixed frequency which is different from each other so as todetune or spread excitation energy out in the frequency spectrum. Thisavoids excessive excitation of hardware structure frequencies. Further,in another embodiment, different sets of the devices 104 can be fired atdifferent frequencies. For example, in a configuration with six devices104, two operate at a first frequency, two others operate at a secondfrequency and the remaining two operate at a third frequency.

Alternatively, additional embodiments can have active frequencymodulation, as discussed above regarding FIG. 1, to provide frequencyavoidance control and detuning. In such an embodiment, each of the pulsedetonation devices 104 can be controlled so that the frequency changesare random (as discussed with regard to FIG. 1), or can be controlledsuch systematically (as discussed above regarding FIG. 1). Moreover, inan additional embodiment, the control of the pulse detonation devices104 is a combination of the random and systematic control methods. Inthis embodiment, the operational frequency of at least one of thedevices 104 is changed/controlled randomly, while the remaining devices104 are controlled systematically. For example, the remaining devices104 are controlled such that the operating frequency is changed in 1 Hzincrements in the range of 195 Hz to 205 Hz.

In a further embodiment of the present invention, the range of operatingfrequency of the pulse detonation device 104, or devices (depending onthe specific configuration) is changed based on the operating conditionof the engine. For example, in a first engine operating condition theoperating frequency range of the device 104 is between 195 and 205 Hz,and in a second engine operating condition the operating frequency rangeof the device 104 is between 165 and 175 Hz.

Although the above discussion has been primarily directed to the use ofthe present invention in conjunction with aircraft engines, those ofordinary skill in the art will recognize that the present invention maybe used with any radial or axial turbine structure, which is driven by apulse detonation device, and is not limited to only use in aircraftengine applications.

Further, while the invention has been described in terms of variousspecific embodiments, those skilled in the art will recognize that theinvention can be practiced with modification within the spirit and scopeof the claims.

1. An engine, comprising: at least one pulse detonation device, whereinan operational frequency of said pulse detonation device is continuouslychanged within a range of available operating frequencies.
 2. The engineof claim 1, wherein the operating frequency of said pulse detonationdevice during a first pulse and the operating frequency of said pulsedetonation device during a second pulse are different, wherein saidsecond pulse directly follows said first pulse.
 3. The engine of claim1, wherein the change of said operational frequency within said range ismade randomly.
 4. The engine of claim 1, wherein the difference betweenany one operating frequency and any consecutive following operatingfrequency is at least 0.1 Hz, and wherein both of said one operatingfrequency and said consecutive following operating frequency are withinthe range.
 5. The engine of claim 1, wherein the difference between anyone operating frequency and any consecutive following operatingfrequency is at least 1 Hz, and wherein both of said one operatingfrequency and said consecutive following operating frequency are withinthe range.
 6. The engine of claim 1, wherein the at least one pulsedetonation device is positioned upstream of a turbine section withinsaid engine.
 7. The engine of claim 1, wherein the operating frequencyis changed to another operating frequency within said range after apredetermined number of pulses of said at least one pulse detonationdevice.
 8. An engine, comprising: a plurality of pulse detonationdevices, wherein an operational frequency of at least one of said pulsedetonation devices is continuously changed within a range of availableoperating frequencies.
 9. The engine of claim 8, wherein the operatingfrequency of said pulse detonation device during a first pulse and theoperating frequency of said pulse detonation device during a secondpulse are different, wherein said second pulse directly follows saidfirst pulse.
 10. The engine of claim 8, wherein the change of saidoperational frequency within said range is made randomly.
 11. The engineof claim 8, wherein the difference between any one operating frequencyand any consecutive following operating frequency is at least 1 Hz, andwherein both of said one operating frequency and said consecutivefollowing operating frequency are within the range.
 12. The engine ofclaim 8, wherein the difference between any one operating frequency andany consecutive following operating frequency is at least 0.1 Hz, andwherein both of said one operating frequency and said consecutivefollowing operating frequency are within the range.
 13. The engine ofclaim 8, wherein the at least one pulse detonation device is positionedupstream of a turbine section within said engine.
 14. The engine ofclaim 8, wherein the operating frequency is changed to another operatingfrequency within said range after a predetermined number of pulses ofsaid at least one pulse detonation device.
 15. The engine of claim 8,wherein an operational frequency of another of said pulse detonationdevices is continuously changed within a range of available operatingfrequencies, wherein said range of said another pulse detonation deviceis different from said range of said at least one pulse detonationdevice.
 16. An engine, comprising: a plurality of pulse detonationdevices, wherein an operational frequency of at least one of said pulsedetonation devices different from an operational frequency of another ofsaid pulse detonation devices.
 17. The engine of claim 16, where atleast one of said at least one pulse detonation device and said anotherof said pulse detonation devices is positioned upstream of a turbinesection of said engine.
 18. A method of frequency modulation in anengine which comprises at least one pulse detonation device, said methodcomprising: operating said engine in at least one operating condition;operating said at least one pulse detonation device with said engine;and continuously changing an operating frequency of said at least onepulse detonation device within a range of available operatingfrequencies.
 19. The method of claim 18, wherein the operating frequencyof said pulse detonation device during a first pulse and the operatingfrequency of said pulse detonation device during a second pulse aredifferent, wherein said second pulse directly follows said first pulse.20. The method of claim 18, wherein the change of said operationalfrequency within said range is made randomly.
 21. The method of claim18, wherein the difference between any one operating frequency and anyconsecutive following operating frequency is at least 1 Hz, and whereinboth of said one operating frequency and said consecutive followingoperating frequency are within the range.
 22. The method of claim 18,wherein the difference between any one operating frequency and anyconsecutive following operating frequency is at least 0.1 Hz, andwherein both of said one operating frequency and said consecutivefollowing operating frequency are within the range.
 23. The method ofclaim 18, further comprising directing an output of said at least onepulse detonation device into a turbine portion of said engine.
 24. Themethod of claim 18, wherein the operating frequency is changed toanother operating frequency within said range after a predeterminednumber of pulses of said at least one pulse detonation device.